In recent years, with expansion of the market of solar cells, and expansion of demand of touch panels due to rapid spread of smartphones and tablets, a transparent electrically conductive film has widely been used as a transparent electrode. As for the transparent electrically conductive film, from a view point of weight saving, film thinning, and increase in flexibility, transparent electrically conductive films have often been used. Currently, most of the transparent electrically conductive films are ITO films in which indium tin oxide serves as an electrically conductive layer.
However, the ITO film has a problem in color tone due to a low light transmittance in a long wavelength region. Moreover, since ITO is a semiconductor, there is a limitation on increase in electrical conductivity, and, since an electrically conductive layer is ceramics, the ITO film has a problem in bending property. For this reason, there has been a demand for a flexible film having a higher transmittance and a high electrical conductivity.
Then, currently, as a next-generation transparent electrically conductive film, various transparent electrically conductive films made from a carbon nanotube, or a metal nano-material such as a metal fine wire and a silver nanowire constituting a mesh structure have been proposed (Patent Literatures 1 to 3).
When the carbon nanotube is used, since the electrical conductivity of the carbon nanotube serving as an electrically conductive filler is inferior to that of a metal material, performance as a transparent electrically conductive film is not satisfactory. In addition, a transparent electrically conductive film formed of a metal mesh structure has a high electrical conductivity, but has a problem that a metal fine wire is visible. A transparent electrically conductive film made from a metal nanowire can realize both of an electrical conductivity and a transparency.
As the metal nanowire used in the transparent electrically conductive film, from a view point of an electrical conductivity, silver is widely used. However, silver is a metal material that has a high electrical conductivity but very easily causes ion migration. Thus, silver adversely influences on the insulating property between film substrates or between wires. In a nanostructure such as a nanowire, even a minute change in shape remarkably changes the electric property. For these reasons, a transparent electrically conductive film made from a silver nanowire as an electrically conductive material has a problem that a reliability of a device or the like is reduced.
Then, there has been proposed a method of imparting an ion migration resistance to a silver nanowire surface by coating the surface with another metal material through plating or the like to improve stability (Patent Literatures 4 and 5). However, in these methods, since the resulting silver nanowire surface is plated or sulfurized, the process is complicated. Moreover, since the different metal is formed only on the surface, there are problems in uniformity, durability, and an electrical conductivity.
For this reason, as for a nanowire made of a metal material other than silver, particularly, there has been proposed a technique concerning a nickel nanowire and a cobalt nanowire, which have high stability and can also be used as a magnetic material (Patent Literatures 6 to 8).
However, in these methods, a metal nanowire is made inside the carbon nanotube. Thus, the methods have a problem that a metal nanowire cannot be used alone, treatment at a higher temperature is necessary (Patent Literature 6), and only a relatively short nanowire having a length of up to around 10 μm is obtained (Patent Literatures 7 and 8).
There has been proposed a method of obtaining a ferromagnetic metal nanowire having a large nanowire length (Patent Literatures 9 and 10). However, nanowires obtained by these methods are not supposed to be dispersed. Thus, the nanowire length is large, but nanowires are entangled complicatedly and come into a state where the nanowires are aggregated in a sheet or cotton form. For this reason, the nanowire obtained by the methods is suitable for use in a structure such as a battery electrode material, but is difficult to be defiberized while maintaining the nanowire shape, and it is impossible to apply the nanowire to paints and inks as a dispersion.
There has been proposed a method of obtaining a dispersed ferromagnetic metal nanowire (Patent Literature 11). In this method, a dispersion in which metal nanowires having a length of around 100 μm are dispersed in a solvent is obtained. However, in the dispersion, nanowires are easily precipitated and aggregated. Moreover, in order that a nanowire film obtained from the dispersion has an electrical conductivity, plating with a noble metal is indispensable. Additionally, since the nanowire making method is of a template system, the method is poor in productivity.